For example, as a light source for optical transmission apparatus or information storage apparatus, a distributed feedback (DFB) laser using a refractive-index-modulation diffraction grating, which has narrow spectrum and allows single mode oscillation, is mainly adopted. In a DFB laser, light output and modulation characteristic significantly change according to κL, product of a coupling coefficient κ of light diffracted in a waveguide direction and an oscillator length L. Therefore, in designing and manufacturing laser devices, it is important to set κL at a desired value. Here, the coupling coefficient κ is determined by height of the diffraction grating, distance from an active layer, and difference in refractive index between a diffraction grating layer and a buried layer (clad layer). In particular, the coupling coefficient κ largely depends on the height of the diffraction grating.
A conventional process of forming a diffraction grating is described below. On an n-InP substrate, an n-InP first clad layer, an n-InGaAlAs first optical guide layer, an InGaAlAs active layer, a p-InGaAlAs second optical guide layer, a p-InP spacer layer, a p-InGaAsP diffraction grating layer, and a p-InP cap layer are formed through crystal growth, such as metal organic chemical vapor deposition (MOCVD). In order to increase a carrier confinement effect, the InGaAlAs active layer includes a multiple quantum well (MQW) having an InGaAlAs barrier layer and InGaAlAs well layer laminated in a periodic structure.
Furthermore, on the p-InP cap layer, an insulating film, such as a silicon dioxide (SiO2) film or a silicon nitride (SiN) film, is formed. Then, through photolithography and interference exposure or electron beam (EB) exposure, a striped pattern is formed in a direction perpendicular to a waveguide. The insulating film is removed through dry etching with fluorinated gas or wet etching with hydrofluoric acid solution, using the resist pattern as a mask. Then, the resist pattern is removed with solvent. Using the insulating film as a mask, the p-InP cap layer and the p-InGaAsP diffraction grating layer are removed through dry etching or wet etching to form a rectangular diffraction grating. Next, a p-InP second clad layer regrowth is performed through MOCVD or the like.